This is a mountable engine created in RailsCasts episode #277. This is not a gem so these instructions will not work, but you can use this as an example readme for your own mountable engine.
First add the gem to your Gemfile and run the bundle command.
Next, mount the engine in your routes.rb under the path you want.
mount Uhoh::Engine => "/uhoh"
Then copy over the migrations and run them.
rake uhoh:install:migrations rake db:migrate
Now when an exception is raised in your application it should be browsable at the “/uhoh” path. Don't forget to password protect if you're using this in production.